Synchrotron radiation, a basis of modern astrophysics
Richard Wielebinski
Max-Planck-Institut für Radioastronomie, Bonn
The classical case of a relativistic electron emitting in a magnetic field was treated by
G.A.Schott in 1912 already. Synchrotron emission was detected in the new electron
accelerators in the 1940s as blue light seen when the beam entered the Earth’s magnetic field.
This led to considerable activity in the studies of synchrotron emission; notably by
J.Schwinger in the USA. The serendipitous discovery of cosmic radio waves by K.Jansky in
1932 at first had no explanation. Also the extremely intensive Solar radio waves, observed by
war-time radar stations, defied explanation. Only in the late 1940s the connection between
cosmic radio waves and synchrotron emission could be established. This was preceded by the
discovery of magnetic fields in the Milky Way. The synchrotron emission requires such
magnetic fields and is observable as polarized emission. The rapid growth of radio astronomy
is directly a result of the fact that cosmic objects emit intense radio waves through the
synchrotron emission process. The radio waves come not just from nearby objects but from a
huge variety of cosmic sources, many at the edge of the observable Universe. Radio galaxies
are synchrotron sources emitting as a result of jets that are powered by active galactic nuclei
interacting with the intergalactic medium. Supernovae leave remnants where the interaction of
a magnetic field with the interstellar medium is obvious. The Milky Way is a giant
synchrotron emitter. Quasars are often first identified as weak synchrotron radio sources
before they are fixed in the distance by optical spectroscopy. In this lecture I will describe the
most recent advances in our knowledge of the cosmic Universe, thanks to the synchrotron
emission process.
Synchrotron radiation is the most important emission process in astrophysics, but it
was a long and difficult way to make it acceptable to astronomers who were used to positional
astronomy, stellar interiors and at most thermal emission from the interstellar medium. The
detection of radio waves of extraterrestrial origin by K.Jansky in 1932 was widely publicized
by the daily press but not taken up seriously by professional astronomers. The Bell Telephone
Company engineer K.Jansky realized that the radio emission was connected in some way with
the Milky Way. The next lone investigator, Grote Reber, followed up these early detections
and started to investigate the radio emissions at different frequencies. Reber concluded that
the observed radio emission could not be due to black-body (thermal) radiation from the
interstellar medium. Reber also discovered that the maximum of the radio emission came
from the direction of the constellation Sagittarius, some 30° away from the then accepted
centre of the Milky Way. Solar radio emission was discovered by war-time radar station
operators in the 1940s who were surprised at the intensity of the radio waves. It became also
clear that solar radio emission was highly variable, correlated with visible eruptions and with
the solar cycle. All the observed radio emissions called for a new interpretation.
The classical theory of an electron emitting in a uniform magnetic filed was treated by
Schott in 1912 already. Synchrotron emission was detected in the new electron accelerators in
the 1940s. The theory of synchrotron radiation was developed by J.Schwinger in the USA and
a strong group of Soviet theoreticians. The theoretical explanation of the origin of the Solar
radio emission was given by A.Unsöld who concluded that the steady radio emission of the
Sun may be due to free-free (thermal) transitions in the solar corona and the chromosphere but
the variable radio emission of the Sun must be non-thermal, some sort of ‘plasma oscillation’.
The radio emission of the Milky Way was to Unsöld of the same nature as the variable solar
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emissions. At this stage of observational progress ‘radio stars’, strongly emitting cosmic
sources were discovered and it was suggested that this new population of celestial objects
may be responsible for the cosmic radio emission. Theoretical considerations by H.Alfvén
suggested that radio stars may be radio emission generated by electrons in magnetic fields, a
suggestion that pointed to the presence of synchrotron radiation in cosmic sources.
K.O.Kiepenheuer went into the details of possible scenarios, and concluded that radio stars
are less likely to be a source of the ‘background emission’ but that interstellar space, when
filled with magnetic fields, could generate the observed radio intensities. By 1950 the basic
explanation was given and waited for observational confirmation. The understanding of
polarization effects of synchrotron emission was paramount in the following development
since it offered new ways of observing. The most concise reviews about synchrotron radiation
in astrophysics are due to Viatli Ginzburg and A.G.Pacholczyk.
Magnetic fields have been detected in Solar sunspots by G.E.Hale in 1908 by
observing the Zeeman splitting of optical spectral lines. Improved sensitivity allowed the
detection of magnetic fields in ‘magnetic’ Ap stars using the same method. In 1949 optical
polarization observations by W.A.Hiltner and J.S.Hall were published. In 1951 L.J.Davis &
J.L.Greenstein developed an interpretation that suggested that the optical polarization was due
to dust grains aligned by magnetic fields of the Milky Way. Thus one of the requirements for
synchrotron radiation, the presence of cosmic magnetic fields, had been established.
.
The mysterious ‘radio stars’ were identified by J.Bolton in 1949 with such diverse
cosmic objects as a historical supernova remnant (Crab nebula), a nearby galaxy (Centaurus
A) and a distant galaxy with a jet (Virgo A). This step was very important, because it
established the radio band as an important astronomical window. The Crab Nebula turned out
to be the ‘Rosetta stone’ of astrophysics, leading to the establishment of the synchrotron
theory as a basis of modern astrophysics. Further observations in the northern sky discovered
such intense radio sources as Cassiopea A (a supernova remnant) and Cygnus A (a radio
galaxy). Sky surveys continued to be made giving us lists of 100s of sources, notably the
Cambridge 3C survey and the surveys of B.Y.Mills in Australia. Observations of radio
sources at different frequencies led to the determination of the spectral index that confirmed
the synchrotron nature of these cosmic objects.
The Crab nebula played the crucial role in the progress of understanding the radio
emission, a triumph of the synchrotron theory. I.Shklovskii suggested that the emission
(optical and radio) of the Crab Nebula is due to the synchrotron process. This was motivated
by observations of linear polarization of the Crab nebula optical emission made very early by
Soviet astronomers. The search for the radio polarization was a logical consequence. After
several negative attempts a detection of linear radio polarization at high radio frequencies was
made in 1957. Another object that was important in the earliest days was Virgo A (M87).
This is a peculiar galaxy with a jet and a very intense radio emission. G.R.Burbidge
proceeded to interpret both the optical and radio emission to be a result of synchrotron
radiation in strong magnetic fields. This object became the prototype of a ‘radio galaxy’
where due to an active galactic nucleus (AGN) intense radio (synchrotron) emission results in
the outer lobes. To support this argument, the discovery of linear polarization in the radio
galaxy Cygnus A in 1962 was most important. Further surveys of the polarization of radio
sources were made, indicating the presence of synchrotron emission everywhere.
. In addition the diffuse Galactic emission, originally observed by Jansky and Reber, was
studied indicating that it was not a superposition of individual discrete radio sources but
extended ‘background emission’ (in fact a foreground emission) originating in the interstellar
medium of the Milky Way. Due to the position of the Galactic centre at southern elevations
(in fact nearly overhead in Sydney, Durban and Buenos Aires) a combination of surveys from
both Northern and Southern observatories became important. At first these radio maps were
made at low radio frequencies with moderate angular resolution. The spectrum of the diffuse
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radio emission also confirmed the emission to be due to synchrotron radiation. The detection
of polarisation of diffuse Galactic emission proved to be more difficult. Many attempts were
made but only after technical innovations were implemented by Wielebinski and Westerhout
in 1962 the first unambiguous detection of the Galactic linear polarization was published.
These observations showed that linear polarization was detectable all over the (northern) sky,
hence telling us that magnetic fields and the synchrotron radiation process were most widely
spread everywhere in the Milky Way. Further observations showed that polarized radio waves
were rotated by the passage through the magneto-ionic medium (Faraday effect) in the
ionosphere and in the Galactic interstellar medium. These observations added another method
of studying the magnetic fields in the line of sight. They also pointed out that passage of the
polarized radio waves through the Galactic medium had to be corrected by multi-frequency
observations. Since most radio sources are polarized (the only exception being thermal HII
regions) they can be used to probe the magnetic fields between the source and the observer.
The origin of the observed magnetic fields has been unknown. Early work of Ludwig
Biermann in 1950 suggested a ‘battery’ mechanism for the generation of magnetic fields.
However the intensity of ‘battery’ fields was negligible. The development of the Dynamo
theory by Parker, Krause, Steenbeck and others offered the possibility of field amplification
by substantial factors. One of the requirements of the theory, the existence of ‘seed fields’,
was missing. Recent work suggests that the early cosmic universe or plasma instabilities may
provide the seed field necessary to account for the observed field strengths in cosmic radio
sources, thus closing the loop in interpretation. In addition extensive magnetohydrodynamic
simulations became possible showing the effects of magnetic fields on interstellar matter.
The majority of radio sources in the cosmic universe emit by the synchrotron process,
especially at the lower radio frequencies in the metre and cm wavelength range. Most of the
sources are linearly polarized to a degree of 1% to 75%. The highest polarization is observed
in thin regions away from depolarizing thermal clouds (depolarization due to the Faraday
effect). The observations of linear polarization can be used to determine the magnetic fields in
the emitting objects. The increase in the angular resolution and better sensitivity of modern
radio telescopes helps in delineating the polarized regions. Circular polarization is much
lower, 0.1% – 2%, seen in some special sources like BL lac objects, Active Galactic Nuclei,
etc. Other forms of radio emission are thermal emission from hot gas clouds or from cold dust
and maser emission in e.g. molecular clouds or pulsars. Synchrotron emission has been
detected from the most distant quasars, implying the presence of magnetic fields in the early
universe and making it the most important probe of cosmic evolution.
The data on the synchrotron radiation in astrophysics have gone through an enormous
development. While at the beginning of observations in the 1950s angular resolution of the
single dish radio telescopes was counted in degrees, we now have observations with the
resolution of sub-arc seconds done with aperture synthesis arrays. Radio astronomers can
compete in angular resolution with the Hubble Space Telescope. The technique of very long
baseline interferometry (VLBI) has taken the angular resolution down to milli-arc seconds
and is posed to improve this to micro-arc seconds. This allows radio astronomers to probe the
inner nuclear regions of galaxies. The sensitivity of the radio observations, which started with
the detection of sources of several JANSKY (1 Jy = 10 -26 watts meter-2 Hz-1), has trough
improvements of the receivers and telescopes now reached the sensitivity in the microJy
range. The frequency coverage of radio telescopes now spans a huge spectral range from
meter to sub-mm wavelengths. New radio telescopes like the ALMA and LOFAR (under
construction) or the SKA project (planned) will improve the angular resolution even further at
the same time offering huge increase in sensitivity. We can expect more exciting observations
as a result of these new instruments and the synchrotron emission process.
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